Use IZooKeeper instead of ZooKeeper directly to permit mocking.

Using the concrete ZooKeeper class instead of the IZooKeeper interface
was preventing use of mock frameworks like Rhino.Mocks to mock the
ZooKeeper service when testing code leveraging recipes.
Merge pull request #9 from tausten/trunk

Use IZooKeeper instead of ZooKeeper directly to permit mocking.